ERR_NGROK_3904: The body content could not be successfully modified for action type ACTIONTYPE - ngrok documentation

What does ERR_NGROK_3904 mean?

ngrok returns ERR_NGROK_3904 with this message:

The body content could not be successfully modified for action type <ACTIONTYPE>

Where can I get more help with ERR_NGROK_3904?

Email support@ngrok.com and include the error code.